Synthetic leather typically is produced by coating or laminating an elastic polymer resin, such as a polyurethane resin, a polyvinyl chloride resin, or a blend of such resins, onto the surface of a fibrous base material. In order to produce a synthetic leather in a variety of colors and/or shades, various pigments or dyes have been used to color the resin applied to the surface of the base material. However, the use of such pigments and dyes has not been without its disadvantages and drawbacks.
For example, pigments usually have low tinting strength and a dull shade, which can limit the aesthetic qualities of synthetic leather produced therefrom. Pigments typically lack solubilizing groups, which frequently allows the pigment particles to aggregate and form larger secondary and tertiary aggregate particles during production processes. Owing to these difficulties, synthetic leathers colored with conventional pigments often exhibit poor color retention, have a dark or dull shade, or contain unsuitable variations in color depth. While these problems can be partially addressed through the addition of dispersing agents or by utilizing pigment dispersions, these measures often result in increased production costs and require great care to minimize color variations produced by settling of the pigment(s) and/or incompatibility of these components with the resin.
Dyes, on the other hand, typically contain solubilizing groups that can facilitate dispersion of the dye in a suitable medium. Dyes also typically exhibit relatively high tinting strength, good transparency, good thermal stability, and acceptable resin compatibility. Nevertheless, dyes typically exhibit poor weather durability, poor water resistance, poor oil resistance, and often migrate or bleed through to the transfer substrates, such as a release paper, used to produce the synthetic leather, and to other substrates in contact with the synthetic leather surface. In order to address the migration of dyes to the transfer substrate, attempts have been made to utilize nylon or polyurethane overcoats applied to the transfer substrate. However, satisfactory topcoats have not been developed without incurring significantly higher cost.
U.S. Pat. No. 7,662,461 to Xia and U.S. Pat. No. 7,824,737 to Xia disclose synthetic leather articles comprising a polyurethane coating made from polymeric colorants and/or polyurethane colorants made from polymeric colorants with reactive functional groups. Such polyurethane polymeric colorants give great transparency, compatibility and vivid colors. But these polyurethane colorants do not have good migration properties as pigments, or as good as desired.
A need therefore remains for novel colored synthetic leather articles that address the deficiencies of articles produced with conventional pigments, dyes and/or polymeric colorants, while still exhibiting the desired aesthetic qualities. The present invention provides such articles and methods for producing the same. This invention intends to provide colored polyurethane synthetic leathers which are transparent, compatible with the polyurethane resins, and do not have migration issues to release papers and/or other contacting substrates and surfaces.